Standoff threat detection provides the capability to detect trace amounts of explosive, chemical, biological, or other agents without physical contact, obviating the need to obtain consent or cooperation from targets. Operation of detection equipment may be carried out at a significant distance, or up close to an object of interest, for example, from 0.5 to 100 meters away, reducing the level of risk posed to security personnel.
Such techniques are possible because the process of manufacturing threats, such as explosives, typically results in trace residues including particles of the threat material being left on persons and objects associated with manufacturing process. IR-laser spectroscopy and other techniques may be used to detect trace amounts of chemicals, such as explosives, which typically exhibit strong absorbance patterns in the mid-IR region. In order to calibrate the equipment to accurately detect trace amounts of threat materials and establish limits of detection for various threats, reference materials and standards are needed. The reference materials and standards may also be used to develop new techniques and new equipment to more accurately detect threats. Standards having known areal density are also desired, and control over variables including particle size, shape, and areal distribution would be beneficial. Given the very small amounts and volatility of the materials involved, production of consistent standard materials is difficult.
Techniques that may be used to deposit particles for detection, calibration, or further testing include inkjetting, aerosol spraying, pipetting, and spin coating onto a substrate.
Inkjetting has been popularized because of its use in printing, and has been adapted to print other chemicals of interest as a solution which dries on contact with an intended surface. Inkjetting is largely limited in its control of particle deposition by the solvent selected and its interaction or wetting with a surface. As the solution dries, complex processes can occur which may result in a wide variety of results, including coffee ring features, dome structures, isolated arrays of sub-micron sized particles, polycrystalline structures, and more. None of these represent typical particles found in fingerprints because their sizes and groupings do not match what is produced in an actual fingerprint. In addition, because drops deposited by inkjetting are typically very small, they evaporate very quickly and may trap substantial amounts of solvent in the produced crystal. The shapes, distribution, and content of particles is important in standoff detection technologies because the optical spectroscopies employed are often affected by the range of particle sizes involved and their chemical content. Further, inkjetting is inherently a serial process.
Sieving stacks are routinely used in laboratories to fractionate particles with one or more sieves. A collection pan is provided at the base of the stack in order to collect the processed particles. The stack is typically held together using a rigid support structure, and the base of the stack is positioned on a vibratory plate to couple a vibratory action to the sieving stack. The vibration causes the particles to move within each sieve and through suitable sieve openings into the receiving collection pan. An impact hammer action can also be applied to the top of the stack, and is typically operated at 1 Hz to assist the sieving action and reduce mesh pore blockages. The sieving stacks are typically operated in ambient air, which limits their practical use to particles of about 20 microns and above.
Using the conventional sieving stacks, the sieving of particles to a loading of between 10-100 micrograms/cm2 can take tens of minutes, which is time consuming if multiple sieving operations are needed. In addition, there is no means to compensate for non-uniform sieving through a sieve membrane. Non-uniform sieving can result because of pore opening blockages, vibration nodes that form in the sieving membrane causing particle pooling in a sieving pan, and any sieving membrane/pan slope that leads to particle pooling towards one side of a sieving or collection pan. Feedback control to stop particle deposition when a desired amount of particles have been deposited is also lacking. These limitations present significant drawbacks when applying sieving or other techniques to fabricate samples or coupons which require controlled particle amounts and controlled particle distributions.
U.S. Pat. No. 9,022,220 describes a sieving system that includes a filter, a blade stirring a powder accumulated on the filter, a driver driving the blade, and a notifier for notifying a user of predetermined information regarding the status of the filter, based on a load on the driver while driving the blade.
U.S. Pat. No. 8,973,759 describes a sieving device including a hollow cylindrical body, a filter disposed at a bottom portion of the hollow cylindrical body, and a blade configured to rotate in close proximity to the filter around a rotation axis thereof, crossing the filter to thereby stir powder supplied to the hollow cylindrical body.
Each of these techniques uses a blade to stir and mix a powder present in sieve, and monitors the sieve for blockages.
However, none of the existing deposition techniques are able to adequately address the problems associated with non-uniform particle deposition, particularly in the context of standoff detection applications.